Evaporator defrost by means of electrically resistive coating

ABSTRACT

A defrosting system for defrosting an evaporator assembly is disclosed. The system includes the evaporator assembly, an electrically resistive coating having an electrically insulative matrix and a conductive doping agent disposed on at least one surface of the evaporator assembly, and a plurality of electrical terminals arranged and disposed to supply electricity to the electrically resistive coating. A method for defrosting an evaporator assembly and a coating for heating evaporator assemblies are also disclosed.

FIELD OF THE INVENTION

This invention relates to automatic defrost technology for refrigerationequipment, in particular, defrosting refrigeration evaporator coils andassociated conductive fins by means of an electrically resistivecoating.

BACKGROUND OF THE INVENTION

In standard refrigeration equipment, the heat absorbing element of thecooling technology and other cooled surfaces will continually accumulateliquid condensate or frost from atmospheric moisture rendering thesystem less efficient and inconvenient to maintain. A variety ofautomated defrost technologies are employed to eliminate frost buildupbut these generally require heating the surfaces for a brief period thusraising the air and product temperature within the freezer. For somedevices, this temperature variation exceeds the acceptable limitsrequired to maintain product viability.

In the area of refrigeration, the typical defrosting cycle is achievedthrough the heating of a discrete electrical heating element in closecontact or in the vicinity of the evaporator element or by means ofbypassing hot refrigerant from the condenser circuit which is connectedto the evaporator in a similar manner to a typical discrete electricalheating element. There are also a variety of additional methodologiesemploying variants of these methods. These types of systems tend to beinefficient with energy utilization and unequal in energy distributionapplied to the melting or sublimation of ice.

There is not found in the prior art a method for the utilization ofachieving a conformal coating electrically resistive heating elementthat would allow for high levels of evaporator area coverage and highlyuniform heating.

The disclosed method utilizes a conformal coating applied to anevaporator that is electrically resistive and can thus be utilized as adefrosting element with the advantages of high surface area coverage andhigh heat distribution uniformity.

SUMMARY OF THE INVENTION

Embodiments of the present invention include the controlled usage of anelectrically conductive coating to melt or vaporize frost accumulationor ice buildup.

Other embodiments of the present invention include a defrosting systemfor defrosting an evaporator assembly. The system includes theevaporator assembly, an electrically resistive coating having anelectrically insulative matrix and a conductive doping agent disposed onat least one surface of the evaporator assembly, and a plurality ofelectrical terminals arranged and disposed to supply electricity to theelectrically resistive coating.

Still other embodiments of the present invention include a method fordefrosting an evaporator assembly. The method includes providing anevaporator assembly comprising an electrically resistive coating havingan electrically insulative matrix and a conductive doping agent disposedon at least one surface of the evaporator assembly and a plurality ofelectrical terminals arranged on a surface of the electrically resistivecoating. Electricity is supplied to the electrically resistive coatingto heat ice or water present on the electrically resistive coating.

It is an aspect of the invention to provide an electrically resistiveheating element that conforms to the shape of the evaporator tubing andfin construction.

Another aspect of the invention is to provide an electrically resistiveheating element that conforms to the shape of any cooling elementtechnology including thermoelectric and magnetic types of technologiesand the associated heat exchanging elements.

Still another aspect of the invention is to provide a refrigerationdefrost system that can be adapted for any freezer or refrigerator.

Another aspect of the invention is to provide an electrically resistiveheating element that conforms to the shape of any cooling element thatcan have electrical terminals attached to the conformal coating in avariety of positions to energize the electrically resistive coating.

Still another aspect of this invention is to create a defrost heatingcapability that can be energized for very short durations relative toother technologies which provides the ability to remove frostaccumulation more frequently due to the uniform and high intensityheating capability of certain conductive carbon nanotube-containingelectrically resistive coatings.

Other features and advantages of the present invention will be apparentfrom the following more detailed description, taken in conjunction withthe accompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric illustration showing a standard evaporatorutilizing tube and fin construction also showing electrical terminalconnections in contact with the electrically resistive coating.

FIG. 2 is a front facing illustration showing a standard evaporatorutilizing tube and fin construction also showing electrical terminalconnections in contact with the electrically resistive coating.

FIG. 3 is a side view illustration showing a standard evaporatorutilizing tube and fin construction also showing electrical terminalconnections in contact with the electrically resistive coating.

FIG. 4 is an illustration showing the layers of coating covering anevaporator tube, according to an embodiment of the invention.

FIG. 5 is an illustration showing the layers of coating covering anevaporator fin element, according to an embodiment of the invention.

FIG. 6 is a characteristic set of curves demonstrating the temperaturerise on a coated surface as a function of time for three levels ofelectrically conductive carbon nanotube CNT concentrations.

FIG. 7 is a characteristic curve demonstrating a typical relationshipbetween driving voltage and power dissipated per unit area for a coatedsurface.

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION OF THE INVENTION

The invention generally relates to the field of refrigeration and theability to apply heating energy directly or indirectly to the evaporatorelement within a refrigeration system in order to affect defrostcycling.

Embodiments of the invention function in a superior way compared toother electrical defrost technologies due to the more even and effectivedistribution of active heating elements.

Functionally, a conformal coated evaporator will more uniformly,efficiently, conveniently and quickly remove ice or condensation fromcomplex geometries, such as that of a typical vapor cycle refrigerationsystem evaporator, but the advantages are realized on most geometries.Most existing technologies utilize a resistive element concentrated atone location on or near the cooling element in a refrigeration system.These existing systems rely on free convection and conduction from onelocation to raise the overall temperature of the cooling element. Thiscauses uneven heating, frequently un-melted ice and a generallyinefficient use of energy.

The utilization of a conformal coating allows for a direct applicationof energy to the affected area and a secondary heat transport mechanism(conduction and/or convection) is not required. The heat generated isdirectly absorbed by the ice or water providing the latent heatnecessary for phase change and ultimately the removal of the ice orwater by direct run off, vaporization or accelerated sublimation atlower powers. The application of electrical power to the conformalcoating is multivariant function of the defrosting application.Electrical current can be applied as direct current (DC), alternatingcurrent (AC) modulating voltage, amperage or frequency or as a digitallycontrolled pulse width modulation (PWM) controlling amplitude, pulselength, time-off length and any macro variation in the application ofthe pulse profile over time (i.e. adjusting up or down any of thevariables over a period of time and then reverting to another state).For example, the supplying electricity may include applying electricityfor up to about 30 minutes or up to about 20 minutes or up to about 10minutes or up to about 5 minutes or from about 10 seconds to about 30minutes or from about 30 seconds to about 20 minutes or from about 1minute to about 10 minutes or from about 5 minutes to about 10 minutesor about 10 minutes. In another embodiment, the electricity is appliedfor short time durations to constantly ablate ice crystals as theyaccumulate to prevent ice buildup. For example, the supplyingelectricity may include repeatedly applying electricity as up to about10 microseconds, or from about 1 to about 10 microsecond pulses or fromabout 2 to about 8 microseconds or from about 4 to about 6 microseconds.The repeated pulses in this embodiment may be provided for up to 10pulses or up to 100 pulses or up to 1000 pulses or more. It is throughthe strategic use of these various power application strategies thatmaximum effectivity for condensate evaporation, frost melting, or highfrequency moisture ablation can be affected.

The preferred application of electrical power as described above isdependent on the refrigeration or freezer application and the requiredice or condensate mass removal. To this point the following situationalstrategies are identified:

-   -   1. Timed defrost (heavy frost, ice or liquid condensate        removal)—In this case, the electrically resistive coating 6 is        energized by a continuous direct or alternating current for a        period of time.    -   2. Variable intensity, periodic, timed defrost (heavy to light        frost, ice or liquid condensate removal)—In this case, it is        preferred that the electrically resistive coating 6 is energized        by a continuous direct current at 48V or less modulating a        percentage of time energized using PWM.

Referring now to FIG. 1 , the preferred embodiment of the invention isillustrated with a power supply to the unit as power cord 8. Theevaporator is shown as being comprised of tubes 1, that circulate coldrefrigerant which can be constructed of numerous materials such ascopper, steel, aluminum, other metals, or non-metals, such as plastics,with copper being the preferred material. Attached to the tubes 1, byseparable or inseparable methodologies are a plurality of fins 2, withfins being spaced between 0.06″ to 0.50″ which can be constructed ofnumerous materials, such as copper, steel, aluminum, other metals ornon-metals, such as plastics, with aluminum being the preferredmaterial. Applied to the evaporator assembly 5, is an optionalelectrically insulating coating 4, and an electrically resistive coating6 (See FIGS. 5 and 6 .)

The electrically resistive coating 6, according to the presentinvention, includes an electrically insulative matrix and a conductivedoping agent. The electrically insulative matrix may advantageouslyinclude an electrically insulating polymer and provides a matrix havingelectrically insulative properties. Suitable compounds for use as theelectrically insulating polymer include, but are not limited to,urethane, epoxy, silicone, polyacrylamides, polyvinyl alcohol amongother polymer matrix bases, and combinations thereof. A particularlysuitable electrically insulating polymer includes urethane. Theconductive doping agent includes a material having electricallyconductive properties and, when utilized, in combination with theelectrically insulative matrix, provides resistive heating uponapplication of electricity to the electrically resistive coating 6.Suitable compounds for use as the conductive doping agent include, butare not limited to, carbon nanotubes, graphene, graphite, carbon black,metal, combinations thereof, and other compounds providing similarelectrically conductive properties in an electrically insulatingpolymer. A particularly suitable conductive doping agent includes carbonnanotubes (CNTs). Suitable loadings for the conductive doping agent inthe electrically resistive coating 6 include a loading that provides anelectrical resistance suitable for forming a resistive heating elementupon application of electricity. The loading provides a highly effectiveand homogeneous heating element. The loading concentration of theconductive doping agent may vary from about 0.001 wt % to 70 wt % or 20wt % to about 70 wt % or 30 wt % to about 60 wt % or from about 40 wt %to 50 wt % or from about 0.001 wt % to 50 wt % or from about 0.001 wt %to 40 wt % or from about 0.001 wt % to 20 wt % or from about 0.001 wt %to 5 wt % or from about 0.5 wt % to 5 wt % or from about 1 wt % to 5 wt% or from about 3 wt % to 5 wt % or about 3 wt % or about 4 wt % orabout 5 wt % or about 25 wt % or about 30 wt % or about 40 wt % or about50 wt % or about 60 wt % or about 70 wt % depending on the targetelectrical resistance and heat dissipation per unit area. Likewise, theconductive doping agent is provided in combination with an electricallyinsulative matrix having a compatible chemistry and provides resistiveheating upon application of electricity. In one particularly suitableembodiment, the electrically resistive coating 6 includes from about0.001 wt % to about 5.0 wt % carbon nanotubes as the conductive dopingagent and urethane as the electrically insulative matrix. Anothersuitable embodiment, the electrically resistive coating 6 includes about10 wt % to about 70 wt % carbon black as the conductive doping agent andurethane as the electrically insulative matrix. Another suitableembodiment includes a combination of carbon black and graphite in anamount of about 10 wt % to about 70 wt %. In addition, other conductivedoping agents may be used in place of CNTs or in addition to the CNTs,including, but not limited to, graphene, graphite, carbon black, metal,and combinations thereof. Carbon nanotubes suitable for use with thepresent invention exhibit beneficial electrical, mechanical and thermalconductivity properties. Carbon nanotubes can be synthesized by a numberof methods including carbon arc discharge, pulsed laser vaporization,chemical vapor deposition (CVD) and high-pressure carbon monoxidevaporization. Of these, carbon nanotube synthesis by CVD can providebulk production of high purity and easily dispersible product. Othermaterial variants of carbon nanotubes may be utilized. The carbonnanotubes can be any of single wall carbon nanotube, double wall carbonnanotube, multiwall carbon nanotube, or a mixture thereof, length,diameter, and chirality can vary according to processing methods,duration and temperature of the synthesis.

In one embodiment, the electrically resistive coating 6, according tothe present invention, is formed by applying a coating of urethane-basedcarbon black containing composition, such as HeetCoat (available fromSmartPaint, wwwsmartpaintsolutions.com), to the desired evaporatorassembly 5 and permitting the composition to dry.

In addition, one or more additives may be provided to one or both of theelectrically insulating coating 4 and the electrically resistive coating6 in an amount of up to 20 wt %, or from about 0.01 wt % to about 10% orfrom about 0.1 wt % to about 5 wt % of the respective coatingcomposition. Suitable examples of additives for use in embodiments ofthe present invention include, but are not limited to fillers,colorants, polymer additives (e.g., plasticizers, pH adjustmentadditives, etc.), antioxidants, clarifiers/nucleating agents, flameretardants, light stabilizers or other known additives for polymericcoatings.

The electrically insulating coating 4 can be optional in the case butnot exclusively to the case that the electrically resistive coating 6incorporates an electrically insulating property inherent to the coatingtechnology. Also, on the assembly are a plurality of electricalterminals 3 made from copper sheet or copper wire that are used tosupply electricity to the electrically resistive coating 6. This will,in turn, cause an elevation in the temperature of the electricallyresistive coating 6. The electrical current applied can be either asdirect current or alternating current. This is dependent as stated abovein sections 1 and 2.

Referring to FIG. 4 we now demonstrate typical representative crosssections of the evaporator assembly 5, tube 1, and fin 2, showing theapplied optional electrically insulating coating 4, and the electricallyresistive coating 6.

The enablement of the invention requires the application of theelectrically insulating coating 4 and the electrically resistive coating6 and the electrical terminals 3, used to energize the coating whenelectrical current is applied. The controlled application is achievedusing conventional aerosolization and spraying equipment to apply layersin a controlled manner. In a typical application, the evaporatorassembly 5 would be prepared using cleaning solvent or other method toensure the surface will provide proper adhesion for the coatings.Cleaning can be achieved by spraying or immersing in weak acidic acid,manually scrubbing and rinsing with deionized water.

Once the evaporator assembly 5 is cleaned, it is positioned such thatthe areas of application are easily accessed. The application ofcoatings may be such that the evaporator assembly 5 is covered greaterthan 98%, or greater than 95%, or greater than 90%, or greater than 85%of the surface area of the fins and tubes. This level of coverage willallow for conducted heat to melt ice on adjacent, uncoated surfaces.

If required, the electrically insulating coating 4 is applied to theaffected areas. The coating thickness can vary from 2 to 5 mils as longas the necessary dielectric value of 0.5 kV/mil to 2 kV/mil is achieved.The electrically insulating coating 4 may be applied at any suitabletemperature and may include any suitable electrically insulating polymeror resin, such as polyurethane, polyester, silicone, epoxy, alkyd oracrylate, with polyurethane being the preferred coating. Aerosolizedparticle size control is not critical and most common painting systemseasily achieve the necessary atomization and volumetric applicationrequirements.

Following the application of the electrically insulating coating 4 oralternatively without the electrically insulating coating 4, in the casethat the electrically resistive coating 6 is used. A preferred methoddoes not include the use of electrically insulating coating 4. Thismethod has intrinsic properties necessary to self-isolate from the metalsubstrate (thus achieving effectively a dielectric value of 0.5 kV/milto 2 kV/mil) allowing for the omittance of the electrically insulatingcoating 4. The plurality of electrical terminals 3 are glued or attachedby some other method, such as a compatible pre-applied pressuresensitive adhesive (pressure sensitive adhesive being the preferredmethod in place on the contacting side facing the electricallyinsulating coating 4 or the evaporator assembly 5, directly while aconductive surface on the other side of the electrical terminal 3 isexposed). The positioning of the electrical terminals 3 will bedependent on the geometry of the evaporator assembly. As a general rule,electrical terminals 3 are applied such that the majority of electricalterminals 3 are to be applied to electrically resistive coating 6 byvolume and linear dimension evenly distributed and centered between theelectrical terminals 3. This procedure enhances the uniformity ofcurrent flow through the electrically resistive coating 6 and, in turn,achieves a uniform heating of the electrically resistive coating 6 whencurrent is applied. As a rule, the electrical terminal must be sized andpositioned such that 90% of the end linear dimension is directlyenergized.

Following the application of the electrically insulating coating 4 oralternatively without the electrically insulating coating 4, in the casethat the electrically resistive coating 6 is used. As previously noted,this has intrinsic properties allowing for the omittance of theelectrically insulating coating 4 and the application of a plurality ofelectrical terminals 3. In this case, the electrically resistive coating6 is then applied in the same basic manner as the electricallyinsulating coating 4. The application parameters (atomized particle sizecreation and distribution and the volumetric flow) can vary widelydepending on the conductive particle loading percentage within thecoating and the type and viscosity of the polymeric matrix. Conductivecoatings using carbon nanotube (CNT) conductive particles, basic carbonparticles or other particle types will have unique applicationparameters. The application of the electrically resistive coating 6 mustbe properly controlled to achieve a coverage area and nominal thicknessthat create the resistive properties required for the heatingapplication. In the preferred application, an aerosolized particle sizeof 1 to 2 mils would be typical.

After all application processes are complete for the electricallyinsulating coating electrical terminals 3, and the electricallyresistive coating 6, the evaporator assembly 5 can be installed in theoverall refrigeration system and the electrically resistive coating 6energized accordingly.

In an example referencing FIG. 6 , we demonstrate that practical % CNTloading of a polymer coating (loading can vary dramatically and greatlyaffect conductivity) very high temperatures can easily and quickly beachieved. In FIG. 6 we demonstrate that a temperature rise of 30° F. canbe achieved in less than 2 minutes on an ice coated surface.

FIG. 7 demonstrates the performance of an exemplary CNT conductivecoating dissipating heat as a function of aspect ratio (length to widthratio of the element area between the electrical contact areas). For agiven aspect ratio (length to width ratio) a specific electricalresistance is achieved. Resistances of 2 to 80 ohms per square meter arereadily achieved and can be formulated to best serve the application. InFIG. 7 we demonstrate a readily formulated CNT conductive coating thatcan generate very high-power dissipation per unit area at very lowvoltage. Low voltage in this application is considered to be less than48V.

While the invention has been described with reference to one or moreembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. In addition, all numerical values identified in the detaileddescription shall be interpreted as though the precise and approximatevalues are both expressly identified.

What is claimed is:
 1. A defrosting system for defrosting an evaporatorassembly, the system comprising: the evaporator assembly; anelectrically resistive coating comprising an electrically insulativematrix and a conductive doping agent disposed on at least one surface ofthe evaporator assembly; a plurality of electrical terminals arrangedand disposed to supply electricity to the electrically resistivecoating; and an electrically insulating coating disposed between the atleast one surface of the evaporator assembly and the electricallyresistive coating.
 2. The defrosting system according to claim 1,wherein the electrically insulating coating includes urethane.
 3. Thedefrosting system according to claim 1, wherein the electricallyinsulative matrix includes an insulating polymer.
 4. The defrostingsystem according to claim 3, wherein the insulating polymer is selectedfrom the group consisting of urethane, epoxy, silicone, polyacrylamides,polyvinyl alcohol, and combinations thereof.
 5. The defrosting systemaccording to claim 1, wherein the conductive doping agent is selectedfrom the group consisting of carbon nanotubes, graphite, graphene,carbon black, metak, and combinations thereof.
 6. The defrosting systemaccording to claim 1, wherein the electrically resistive coatingincludes from about 0.001 wt % to about 5 wt % conductive doping agent.7. The defrosting system according to claim 6, wherein the conductivedoping agent comprises carbon nanotubes.
 8. The defrosting systemaccording to claim 6, wherein the conductive doping agent comprisescarbon black.
 9. The defrosting system according to claim 1, wherein theelectrically resistive coating includes from about 20 wt % to about 70wt % conductive doping agent.
 10. The defrosting system according toclaim 1, wherein the evaporator assembly includes at least one fin andat least one tube.
 11. The defrosting system according to claim 10,wherein at least 95% of the area of the at least one fin and at leastone tube are coated with the electrically resistive coating.
 12. Thedefrosting system according to claim 1, wherein the evaporator assemblyincludes copper, steel, aluminum, or a non-metal.
 13. A method fordefrosting an evaporator assembly comprising: providing an evaporatorassembly comprising an electrically resistive coating comprising anelectrically insulative matrix and a conductive doping agent disposed onat least one surface of the evaporator assembly, an electricallyinsulating coating disposed between the at least one surface of theevaporator assembly and the electrically resistive coating, and aplurality of electrical terminals arranged on a surface of theelectrically resistive coating; and supplying electricity to theelectrically resistive coating to heat ice or water present on theelectrically resistive coating.
 14. The method of claim 13, wherein thesupplying electricity includes applying continuous direct or alternatingcurrent.
 15. The method of claim 14, wherein the supplying electricityincludes applying electricity for a predetermined time.
 16. The methodof claim 14, wherein the supplying electricity includes applyingelectricity for about 10 seconds to about 30 minutes.
 17. The method ofclaim 14, wherein the supplying electricity includes repeatedly applyingelectricity as about 1 to about 10 microsecond pulses.
 18. The method ofclaim 13, wherein the supplying electricity includes energizing theelectrically resistive coating with an electrical current applied aspulse width modulation.
 19. The method of claim 13, wherein thesupplying electricity includes energizing the electrically resistivecoating with an electrical current applied at a voltage of 48 volts orless.